JP2011-106377A discloses an engine configuration with respect to a positional relation between a spark plug and an injector disposed in a combustion chamber, and a control method by using the spark plug and the injector based on the engine configuration. In the engine configuration, a distance from a center position of a discharge gap of the spark plug to a center position of the injection hole which is closest to the spark plug and a distance from the center position of the discharge gap to a center axis of the fuel spray which is injected from the injection hole are set within a specific range, respectively.
The control method based on the engine configuration is to apply a high voltage to the spark plug over a fuel injection period after a lapse of a predetermined time from the start of fuel injection from the injector. The fuel injected from the injector under high pressure forms a low pressure part by taking away the surrounding air (entrainment). According to such control method, the low pressure part is formed to which the discharge spark generated in the discharge gap is attracted. Therefore, the ignitability of the air-fuel mixture formed around the spark plug can be improved.
In addition, JP2011-106377A discloses an activation control of an exhaust gas cleaning catalyst at engine start as an application example of the attraction action described above.
The present inventor is considering another activation control of the exhaust gas cleaning catalyst based on an engine configuration different from the one disclosed in JP 2011-106337 A. The engine configuration under consideration is similar to the engine configuration of JP2011-106337A in the positional relation between the injector and the spark plug. However, the engine configuration under consideration is different from the engine configuration of JP2011-106337A in that a tumble flow is formed from intake air supplied into the combustion chamber. More specifically, the spark plug of the engine configuration under consideration is disposed on a downstream side of the injector when viewing in a tumble flow direction. The tumble flow formed in the combustion chamber swirls from the upper part of the combustion chamber downward at an exhaust port side whereas swirls from a lower part of the combustion chamber upward at an intake port side. A control being considered is assuming such a tumble flow.
The control under consideration is performed, under a condition where the tumble flow formed in the combustion chamber, so that start timing of an ignition period by the spark plug is set to a retard side relative to the compression top dead center. In addition the control under consideration is performed so that an injection period of the injector in an expansion stroke overlaps at least a part of the ignition period. According to such injection in the expansion stroke, similar effect as the attraction action in JP2011-106377A can be obtained. That is, the discharge spark generated in the spark plug can be attracted to the low pressure part caused by the fuel spray which is injected from the injector and heads toward the ignition plug. Therefore, the attracted discharge spark can be brought into contact with the fuel spray caused by the injection in the expansion stroke.
However, the control under consideration based on the tumble flow reveals the following problem. That is, when the tumble flow formed in the combustion chamber is strong, the discharge spark generated by the spark plug tends to flow in the flow direction of the tumble flow and the attraction action due to the injection in the expansion stroke is obstructed. For this reason, when the tumble flow is strong, it becomes difficult to bring the attracted discharge spark into contact with the fuel spray caused by the injection in the expansion stroke. Then, when such a situation occurs frequently in the combustion cycles, combustion fluctuation between cycles increase to affect performance of the engine.
The present disclosure addresses the above described problem, and an object of the present disclosure is, when the activation control of the exhaust gas cleaning catalyst is performed based on the engine configuration in which a part of fuel spray from the injector heads in a direction toward the spark plug that is disposed on the downstream side in the flow direction of the tumble flow, and the electrode portion of the spark plug is disposed in an area that is above a contour surface of the fuel spray that comes closest to the spark plug, to provide a countermeasure against the obstruction in the attraction action by the strong tumble flow.
A first aspect of the present disclosure is a control device for an internal combustion engine which is configured to control an engine,
wherein the engine comprising:
an injector which is provided in an upper portion of a combustion chamber and is configured to inject fuel into a cylinder directly;
a spark plug which is configured to ignite an air-fuel mixture inside the cylinder by using a discharge spark generated at an electrode portion, the spark plug is provided at the upper portion of the combustion chamber and on a downstream side relative to the injector in a flow direction of a tumble flow which is formed inside the combustion chamber, the spark plug is provided so that a position of the electrode portion is above a contour surface of a fuel spray injected from the injector toward the spark plug;
a lift amount variable mechanism which is configured to switch a lift amount of an intake cam which is provided in the combustion chamber to drive an intake valve at least between a small lift amount and a large small lift amount; and
an exhaust gas cleaning catalyst that is configured to clean exhaust gas from the combustion chamber,
wherein, in order to activate the exhaust gas cleaning catalyst, the control device is configured to:
control the spark plug so as to generate a discharge spark at the electrode portion over a predetermined period on a retard side of compression top dead center;
control the injector so as to perform an expansion stroke injection on the retard side of the compression top dead center, an injection period of the expansion stroke injection overlaps with at least a part of the predetermined period; and
control the lift amount variable mechanism so that the lift amount of the intake cam is kept in the small lift amount.
A second aspect of the present disclosure is a control device for an internal combustion engine according to the first aspect,
wherein, in order to activate the exhaust gas cleaning catalyst, the control device is further configured to:
control the injector so as to perform an intake stroke injection in the same cycle in which the expansion stroke injection is performed; and
control the lift amount variable mechanism so that the lift amount of the intake cam is changed when a growth rate of the initial flame is lower than a predetermined determination value, the initial flame is generated by a contact between a part of the fuel spray caused by the intake stroke injection and the discharge spark.
A third aspect of the present disclosure is a control device for an internal combustion engine according to the second aspect,
wherein the lift amount variable mechanism is further configured to switch operation angle of the intake cam between a small operation angle and a large operation angle,
wherein the engine further comprises a valve timing variable mechanism which is configured to change valve timing of the intake cam,
wherein the control device is further configured to control the valve timing variable mechanism, when the growth rate of the initial flame is lower than the predetermined determination value, so that an opening period of the intake valve coincides with before and after the lift amount variable mechanism switches the operation angle and the lift amount of the intake cam and so that a closing period of the intake valve approaches to an intake bottom dead center.
A fourth aspect of the present disclosure is a control device for an internal combustion engine according to the third aspect,
wherein the valve timing variable mechanism is further configured to change an overlap period in which an opening period of an exhaust valve provided in the combustion chamber overlaps with the opening period of the intake valve,
wherein the control device is further configured to the valve timing variable mechanism so that the overlap period coincides with before and after the lift amount variable mechanism switches the operation angle and the lift amount of the intake cam.
According to the first aspect, it is possible to select the small lift amount as the lift amount of the intake cam for the activation control of the exhaust purification catalyst. Therefore, the tumble flow formed in the combustion chamber can be weakened as compared with a case where the large lift amount is selected as the lift amount of the intake cam. Therefore, it is possible to prevent the attraction due to the expansion stroke injection from being obstructed by the tumble flow.
According to the second aspect, the tumble flow can be weakened when the growth rate of the initial flame is lower than the predetermined determination value.
According to the third aspect, the opening period of the intake valve can be matched with each other before and after the lift amount variable mechanism switches the operation angle and the lift amount of the intake cam, and the closing timing of the intake valve can be brought closer to the intake bottom dead center. Therefore, it is possible to increase the actual compression ratio after the switches in the operation angle and the lift amount of the intake cam, and to weaken the tumble flow further.
According to the fourth aspect, the overlap period between the exhaust valve and the intake valve can be matched with each other before and after the lift amount variable mechanism switches the operation angle and the lift amount of the intake cam. Therefore, deterioration of emission before and after the switches in the operation angle and the lift amount of the intake cam can be suppressed.